1. Field of the Disclosure
The present disclosure relates generally to an apparatus and a method for switching between receivers according to a characteristic of a received signal in a communication system.
2. Description of the Related Art
The Global System for Mobile communications (GSM) is one of the most commonly known standards for voice and/or data. In the GSM network, voice and/or data is transmitted between a mobile station and a base station as a wireless signal through a physical channel which uses both frequency and time division multiplexing. Specifically, each frequency band (e.g., 200 kHz) is divided into frames having 8 time slots per frame. Depending on whether a voice code is half-rate or full-rate, one or two users are assigned to each time slot. Accordingly, a frequency band can support a maximum of 8 or 16 users corresponding to half-rate or full-rate.
GSM can use Gaussian Minimum Shift Keying (GMSK) modulation. Prior thereto, each symbol takes one of two values which are conventionally expressed as +1 and −1. After GMSK modulation, signals include complex-valued samples each having an In-phase (I) component and a Quadrature-phase (Q) component.
The number of GSM subscribers continues to dramatically increase, however, which causes a resource capacity problem to GSM operators. One recent initiative to increase the capacity of the GSM network is known as Voice services over Adaptive Multi-user channels on One Slot (VAMOS). The VAMOS initiative can double system capacity and can help to optimize spectrum efficiency and the use of scarce radio resources, while reducing the power consumption of a radio base station.
In the VAMOS initiative, a network assigns identical physical resources (i.e., a frequency band and a time slot) to two different mobile stations (e.g., a first MS and a second MS) in the downlink. Two mobile stations are assigned a training sequence pair which is an identical training sequence number from two training sequence sets (e.g., a first set and a second set) orthogonal to each other, and previously known to the two mobile stations. The network determines which of the two paired mobile stations should be assigned higher transmission power on the basis of, for example, the near-far position of the two paired mobile stations and uses an Adaptive Quadrature Phase Shift Keying (AQPSK) modulation technique to introduce power imbalance between the two paired mobile stations.
An optimal receiver for a GMSK modulated signal and that for an AQPSK modulated signal are typically different. The optimal receiver for the GMSK modulated signal is a Downlink Advanced Receiver Performance (DARP) receiver, to which only a training sequence of a desired user is known and external interference is unknown. In contrast, the optimal receiver for the AQPSK modulated signal is a Joint Detection (JD) receiver, to which both training sequences of paired users are known.
Although the DARP receiver is optional for a VAMOS-I signal, the JD receiver is compulsory for a VAMOS-II signal. The JD receiver has much better performance for the VAMOS-I signal than does the DARP receiver.
A direct method for detecting the modulation type of GMSK modulated signal and that of AQPSK modulated signal is to estimate a SubChannel Power Imbalance Ratio (SCPIR) between two users. A given SCPIR value of a GMSK modulated signal is infinite. In an AQPSK modulated signal, a given SCPIR value of a VAMOS-I signal and that of a VAMOS-II signal are in a range of −4 dB to 4 dB and in a range of −10 dB to 4 dB, respectively. When external interference is introduced and received power is low, the accuracy of an estimated SCPIR value dramatically degrades.
Accordingly, there is a need for a method which can efficiently select a receiver for accurately detecting a GMSK modulated signal or an AQPSK modulated signal, and can tolerate the interference and received power that are introduced by a signal received according to the conventional GSM system.